1. Field of the Invention
This invention relates to testing systems. Specifically, the present invention relates to electronic testing systems.
2. Description of the Related Art
Modern electronic systems are implemented with a large variety of circuits and devices (e.g. transistors, logic gates, etc). The circuits and devices are often located in a very small area. In addition, the connections between devices are routed within a small area. With such a large density of devices and connections between devices located in a small area, an industry has developed around testing modern electronic systems. The electronic systems are often deployed in circuit boards such as Printed Circuit Boards (PCB). These PCB boards include a large population of devices and a large population of connections between the devices. As a result, a variety of electronic test systems have developed to test Printed Circuit Boards.
A conventional PCB includes devices and a number of connections between the devices. The connections between devices serve as conduits for carrying electrical current between devices. The conduits for carrying electrical current are often referred to as traces. These conduits are also used in testing the devices and ultimately testing the PCB. The conduits running between devices are typically made of a conducting material such as metal. In addition on both the topside and the underside of the PCB, pockets of the metal material called pads, are formed to provide a point of contact for testing.
A conventional electronic test system includes a fixture and tester electronics. The fixture holds a board under test (e.g. PCB). In addition, the fixture aligns the board under test and provides a mechanism for signals generated by the tester electronics to reach the board under test and then return to the tester electronics for analysis. The fixture is positioned on top of the tester electronics.
Electronic test systems perform test on PCB""s by sending currents through traces on the board under test. The currents are sent from one point on the PCB to a second point on the PCB. Typically, a contact is made with a pad on the underside of the PCB at a first location and a contact is made with a pad on the underside of the PCB at a second location. Current is then generated from the first location through the traces to the second location. A voltage can then be measured at the second location to determine if the traces and the connection to the traces are working properly. Tester electronics generate and measure the current. The tester electronics typically includes software that controls and automates the process.
Conventional electronic test systems typically include a wired fixture or a wireless fixture. In conventional electronic test systems that include a wired fixture, a board under test (e.g. PCB) is placed on a support located at the top of a fixture. A plurality of probes run through the central area of the fixture. The probes are housed in a probe plate. The probe plate keeps the probes in a substantially vertical position, so that the probes can serve as an electrical pathway for test signals. The probes make contact with the board under test at one end and extend within close proximity to the tester electronics on the other end. Wires are then run from the tester electronics to the probes via pins inserted in the probe plate. As a result, an electrical pathway is established from the tester electronics, across the wires, to the probes and then to the board under test. Test signals are then generated by the tester electronics. The test signals run across the electrical pathway and back to the tester electronics along a similar path. The test signals are then analyzed by the tester electronics.
In a wired fixture, the wires provide an electrical pathway for trace currents. The currents run through wires and then through the probes, to the board under test. However, as the number of devices on PCB""s has increased and the sizes of the PCB""s have decreased, it has become difficult to place these wires in such a small area. For example, a PCB that is 16 inch by 24 inch may have 3000 to 4000 devices on the board. As a result, 3000 to 4000 wires may need to be connected from the tester electronics to the probes. This results in an incredible amount of congestion in a very small area. In addition if there is a malfunction, it is very difficult to identify a single dysfunctional wire within the 4000 wires. Therefore troubleshooting becomes a major issue.
As a result, a more modern fixture assembly evolved which attempts to eliminate the need for wires in a fixture. This more recent version of the fixture is often referred to as a wireless fixture. In the more recent version, a fixture houses probes, which are used to engage pads on the underside of a board under test. A fixture PCB or wireless PCB is positioned within the fixture and located on an oppositely disposed end of the probes. The wireless PCB includes a plurality of trace patterns for conducting electrical signals within the PCB between pads on both the topside and underside of the wireless PCB. Contact is made between the tester and the underside of the wireless PCB. As a result, an electrical pathway is established between the tester and the wireless PCB. The test signals are routed through the various trace patterns within the wireless PCB. Probes then make contact with the topside of the wireless PCB and an electrical pathway is established between the wireless PCB and the board under test. Ultimately, using the wireless PCB, an electrical pathway is established from the tester, through the wireless PCB, to the board under test.
In order for a conventional electronic test system to function properly, a good electrical pathway must be established between the tester electronics and the board under test. As a result, the contacts and pathway between the tester electronics, the wireless PCB, the probes and the board under test must be established and maintained. In a conventional electronic test system, force is applied to the board under test and the wireless PCB so that the probes can conduct electricity by remaining in contact with both boards. Once the board under test and the wireless PCB are in contact with the probes, the fixture is able to facilitate the transfer of test signals to the board under test. However if there is spacing between either board and the probes, the test signals may not be conducted or may be conducted and produce incorrect readings.
The contact is maintained between the fixture PCB""s and the fixture probes by applying downward forces on the board under test and upward force on the wireless PCB. The board under test is often placed in a vacuum-sealed area and downward forces are applied by removing the air out of the vacuum-sealed area. When the air is removed from the vacuum-sealed area, the board under test experiences a downward force (e.g. vacuum force). In addition, spring-loaded tester contact points (e.g. pins), which engage the underside of the wireless PCB, create upward forces on the wireless PCB. The fixture probes maintain their contact as a result of the downward force from board under test and the upward force from the wireless PCB.
As a result of the foregoing configuration, a great deal of stress can build up in a fixture. As mentioned above, the wireless PCB is subject to forces pushing upward so that it maintains contact with the probes. In addition, there are also downward forces applied to the board under test when air is removed from the vacuum sealed chamber or in systems without vacuum sealing, forces appear when the board under test is pulled down into position for testing. The upward forces are transferred through the wireless PCB to the probes. In addition, the downward forces are transferred through the board under test to the probes. Therefore, the probes receive both upward and downward forces in the fixture. Since the probes are mounted in the probe plate, these forces are also transferred to the probe plate.
There may be non-uniformity or imbalance between the forces. Probes may not be uniformly distributed in the fixture. As a result, there may be a higher concentration of probes in one area than in another area. The non-uniform distribution of probes result in a non-uniform distribution of forces in the fixture and in the probe plate. In addition, the sum of the upward forces may not equal the sum of the downward forces. When the upward forces do not balance the downward forces the fixture and the probe plate may experience an imbalance in forces. When the fixture and the probe plate experience an imbalance of upward and downward forces or alternatively, if the upward force is distributed in a different manner from the downward force, the probe plate, the board under test and the wireless PCB may ultimately deflect, deform, and possibly fracture.
A deflection of the probe plate, the board under test or the wireless PCB may affect the electrical pathway. For example, a deflection of the probe plate, the wireless PCB or the board under test may produce stress in the probes. This may shift a probe away from its contact with a pad on the board under test or the wireless PCB. The stress may cause probe tips at the end of the probes, which contact the board under test and the wireless PCB to fracture. The body of the probe itself may fracture. Ultimately, the stress conditions in the probes may result in a failure of a probe and as a result incorrect test readings or analysis may result.
In addition, the probes are contained and trapped within the configuration of the fixture; therefore the probe ends only displace a limited amount. Commonly, a probe end presses against a spring placed in a housing that is fixed with respect to the probe plate. As a result, the downward forces on the probes are transferred to the probe plate. In a similar fashion, the upward forces on the probes are transferred to the probe plate. Therefore, the probe plate experiences upward and downward forces. The board under test receives downward forces from the air being removed from the vacuum chamber and upward forces from the probes. The wireless PCB receives upward forces from the spring-loaded pins and downward forces from the probes. Therefore, unbalanced forces appear in the probe plate and both PCBs, as a result, the probe plate and the PCB""s may deflect and deform.
Deflection of the probe plate and the PCB""s ultimately may result in failures in testing. In addition hairline stress fractures in the PCB""s may cause incorrect readings. However, it would be hard to detect whether the incorrect reading were due to the probe plate deflection, a failure in a PCB, a device on the PCB, or a trace between devices on a PCB.
Double-ended are press-fitted into a probe plate. In addition, in some configurations such as floating probe configurations, the probes are allowed some freedom of movement in the vertical direction and the probes experience some pivotal motion around their center axis. The bits located in the probes, which make contact with the board under test and the wireless PCB, are located in a separate cavity and have limited spring resisted displacement. As a result, the forces resulting from a deflection of the board under test are borne by the bit in contact with the board under test and result in a downward force on the probe and the probe plate. In addition, the forces resulting from a deflection of the wireless PCB are borne by the bit in contact with the wireless PCB and result in an upward force on the probe and the probe plate. In addition, if these probes are allowed pivotal movement around their center axis, the probes may swing out of position and the bits may lose contact with pads when forces are applied.
Stresses in the probe plate resulting from forces transferred through the probes; result in deflections in the probe plate. Deflections in the probe plate may result in the misalignment of one or multiple probes with the pads in the boards. In addition, misalignments of the probes may result in structural failures in the probes. Both the misalignment of the probes and the structural failure of the probes may result in the incorrect reading of test signals. Lastly, failure of the probe plate, such as hairline fracture or deformation, impact the probes and as a result, it becomes very difficult to troubleshoot whether there is a problem with the probe plate, the contact of the probe to the board, a bad device on the board under test, or a bad trace on one of the PCBs.
During the operation of an electronic tester system the board under test is removed from the fixture after testing. When the board is removed from the fixture the downward forces applied by pulling the board down into place for testing or removing air from the vacuum chamber, is also removed. As a result a significant imbalance of forces occur in the fixture. The fixture may also be removed from the tester electronics. When the fixture is removed from the tester electronics, the upward forces presented by the tester interface pins are no longer applied to the fixture. As a result, once again, a significant imbalance of forces may occur in the fixture. As mentioned earlier, the imbalance of forces, now caused by changes to the fixture configuration, may result in board deflection, probe misalignment and general fixture failure.
Thus, there is a need in the art for an apparatus that minimizes forces in a fixture. There is a need in the art for an apparatus that more effectively balances forces in the fixture, but still provides an electrical pathway for testing. Lastly, there is a need in the art for an apparatus that reduces forces in a probe plate.
A wireless fixture is presented. Downward forces from a board under test and upward forces from a wireless PCB are balanced and reduced using doubled-ended probes, that are press fitted into a probe plate using one or more concentric rings. Press fitting the double-ended probes restricts horizontal and torque movement of the double-ended probes and concentrates downward forces from the board under test and upward forces from the wireless PCB in the vertical direction. The double ended-probes each include a first bit, which is in contact with the board under test and a second bit with is in contact with the wireless PCB. A spring is in contact with the first and second bits and runs the full length of each double-ended probe. Current is transferred from the wireless PCB, through the first bit, to the spring and its housing, to the second bit and then to the board under test. As upward forces are applied from the wireless PCB and downward forces are applied from the board under test, the two bits are displaced in the vertical direction and the spring is compressed proportionally. As a result, the forces applied by the probes to the board under test and the wireless PCB are balanced and bypass the probe plate.
In one embodiment of the present invention a fixture comprises a probe plate including a plurality of cylindrical openings for receiving double-ended probes. Each of the plurality of cylindrical openings forming a sidewall in the probe plate, the sidewall including at least one indentation. The double-ended probes each further comprise, a socket including at least one concentric ring for press fitting into the indentation. An inner housing is positioned within the socket. The inner housing includes a first end and an oppositely disposed second end. A first bit is located within the inner housing and positioned at the first end of the inner housing. A spring is within the inner housing. The spring is in contact with the first bit and runs axially along the inner housing from the first end to the oppositely disposed second end. A second bit is within the inner housing. The second bit is in contact with the spring. The second bit is positioned at the second end of the inner housing.
A method of managing forces in a fixture is presented. The fixture comprises a board under test subject to a downward force and a wireless printed circuit board. The method comprises the steps of positioning a double-ended probe between the board under test and the wireless printed circuit board. The double-ended probe further comprises a first bit in contact with the board under test, a spring in contact with the first bit and a second bit in contact with the spring on one end and the wireless printed circuit board on another end. The method further comprises balancing forces in the probe by moving the first bit downward in response to the downward force, compressing the spring in response to moving the first bit downward and transferring loading to the second bit in response to compressing the spring.
A second method of balancing forces in a fixture is presented. The fixture comprises a board under test and a wireless printed circuit board subject to an upward force. The method comprises the steps of positioning a double-ended probe between the board under test and the wireless printed circuit board. The double-ended probe further comprises a first bit in contact with the board under test, a spring in contact with the first bit and a second bit in contact with the spring on one end and the wireless printed circuit board on another end. The method further comprising the steps of balancing forces in the probe by moving the second bit upward in response to the upward force, compressing the spring in response to moving the second bit upward and transferring loading to the first bit in response to compressing the spring.
A third method of reducing probe forces in a probe plate is presented. The probe forces include a downward force on the probe plate generated by a board under test and an upward force on the probe plate generated by a wireless printed circuit board. The method comprises the steps of positioning double-ended probes between the board under test and the wireless printed circuit board. The double-ended probes each further comprising a first bit in contact with the board under test, a spring in contact with the first bit and a second bit in contact with the spring on one end and the wireless printed circuit board on another end. The method further comprising the steps of reducing the probe forces in the probe plate by moving the first bit downward in response to the downward force generated by the board under test, moving the second bit upward in response to the upward force generated by the wireless printed circuit board and compressing the spring in response to moving the first bit downward and in response to moving the second bit upward.